Pre-2009 iMac models featured either a white polycarbonate enclosure or an aluminium enclosure. The October 2009 iMac model featured a unibody aluminum enclosure, a version of which can still be seen on the current model. The current iMacs released since October 2012 also feature a much thinner display, with the edge measuring just 5 mm.[1]

The features, price, and case design remained unchanged from the iMac G5. The processor speed, however, according to tests run by Apple using SPEC,[2] was declared to be two to three times faster than the iMac G5.

Alongside the MacBook Pro, the iMac Core Duo represents Apple's first computer to feature Intel processors instead of PowerPC processors. It retained the style, design, and features of the iMac G5.

In early February 2006, Apple confirmed reports of video display problems on the new Intel-based iMacs. When playing video on Apple's Front Row media browser, some 20-inch iMacs (those built-to-order with upgraded video cards) showed random horizontal lines, ghosting, video tearing and other problems. The problem was fixed with a software update.

In late 2006, Apple introduced a new version of the iMac including a Core 2 Duo chip and a lower price. Apple added a new 24-inch model with IPS-display and a resolution of 1920 × 1200 (WUXGA), making it the first iMac to be able to display 1080p content in its full resolution, and a VESAFlat Display Mounting Interface. Except for the 17-inch 1.83 GHz processor model, this version also included an 802.11n draft card.[3]

In August 2007, Apple introduced a complete redesign of the iMac, featuring an aluminum, glass and plastic enclosure. There is only one visible screw on the entire computer, located at the base of the iMac for accessing the memory slots. It has a black, plastic backplate that is not user-removable. The 17-inch model was completely removed from the lineup.

In March 2009, Apple released a minor refresh of the iMac line. Changes included a fourth USB port, replacement of two FireWire 400 ports with one FireWire 800 port, replacement of mini-DVI with Mini DisplayPort, and a slightly redesigned base which is thinner. The exterior design was almost identical to the older Intel-based iMacs. The models were one 20-inch configuration and three 24-inch configurations (instead of two at each screen size as before).[16]

In October 2009, a 16:9 aspect ratio screen was introduced in 21.5" and 27" models, replacing the 20" and 24" 16:10 aspect ratio screens of the previous generation. The back is now a continuation of the aluminum body from the front and sides instead of a separate plastic backplate. Video card options entirely switched to AMD, save for the standard onboard Nvidia card in the base smaller model. The iMac's processor selection saw a significant increase.

The Intel i-series chips are introduced to Mac for the first time on the higher-spec 27-inch models.

Default RAM has also been increased across the iMac range. With the advent of the larger screens, Apple doubled the number of memory slots from two to four. Consequently, the maximum memory capacity was also doubled (to 16GB), and for Intel Core i-series (27-inch), quadrupled, to 32GB.

The 27-inch models of the line became the first to offer Target Display Mode, allowing the iMac to be used as an external display for another Mac computer when connected via Mini DisplayPort, a feature that was extended to the 21.5-inch models onwards with the introduction of Thunderbolt.

The Late 2011 Unibody iMac is also the last model to include an internal SuperDrive.

In October 2012, a new iMac model was introduced that featured a considerably smaller body depth than the previous models, measuring 5mm at its thinnest point, and now without an internal SuperDrive. This was partly achieved by using a process called Full lamination. The display and glass are laminated together, eliminating a 2 mm gap between them. The 21.5 in and 27 in screens remained at their previous resolutions, 1920×1080 and 2560×1440 respectively.

As with the 2009 model, memory has been upgraded; the standard specification is now 8 GB, with the 21.5 in model supporting up to 16GB and the 27 in model supporting up to 32GB. It was reported that the 21.5 in iMac would have non-replaceable soldered memory similar to the MacBook Air and Retina display MacBook Pro[34] though tear-downs show that it uses removable memory but accessing the modules requires ungluing the screen and removing the motherboard.[35] The 27 in version features an access port to upgrade memory without disassembling the display.[36] Apple also upgraded the computers' processors, using Intel's Ivy Bridgemicroarchitecture-based Core i5 and Core i7 microprocessors.

Video cards are now Nvidia as standard. USB 3.0 ports are now included for the first time. The 2012 iMac also features the option of a Fusion Drive which combines an SSD and a conventional HDD to create more efficient and faster storage. Apple also removed the built-in optical drive starting with this iMac generation.

On March 5, 2013, Apple quietly announced an education-only version of the iMac, with less powerful specs for a cheaper price. It includes a 3.3GHz dual-core Intel Core i3 processor, 4GB of memory, a 500GB hard drive and Intel HD Graphics 4000, retailing for US$1,099, $200 cheaper than the base-level consumer iMac.[37]

On September 24, 2013, the 2012 iMac model was updated with 4th-generation Intel Haswell processors and Nvidia 7xx series GPU, promising up to 1.4× improvements in performance. It also has 802.11ac Wi-Fi, which is capable of reaching speeds up to 1300Mbit/s and PCIe-based flash storage, offering up to 1.5× the performance of previous-generation (Ivy Bridge) iMacs. This applies to both the Fusion Drive and pure-SSD options. These became the last iMacs to offer Target Display Mode, as the Retina line introduced the following year are incompatible with the feature due to resolution differences.

At WWDC on June 5, 2017, a refreshed model was added with Kaby Lake processors and Bluetooth 4.2 support.

A Retina Display "5K" model with a resolution of 5120 × 2880 was introduced alongside the previous year's models during a keynote on October 16, 2014. This 27-inch model was given faster Haswell processors and its two Thunderbolt ports were updated to Thunderbolt 2. Secondary storage was also upgraded to a 1TB Fusion drive as standard and video options changed over to AMD Radeon R9 M290X and M295X. In May 2015, a separate, affordable, budget counterpart of the 5K 27-inch iMac was announced with lower specifications. That same day the late 2013 iMac lineup was completely discontinued. Later that year, a 21.5-inch "4K" model with a resolution of 4096 × 2304 was released on October 13, 2015 with older Broadwell processors, as the 27-inch counterparts were upgraded that day directly to Skylake ones. The new rechargeable and wireless peripherals were also introduced that day. In 2017 both 21.5 and 27-inch iMacs was refreshed with newer Kaby Lake Processors, new Thunderbolt 3 (USB-C) ports, and Bluetooth 4.2.